In recent years, as electric tools such as electric drills or electric grinders have grown more and more powerful, battery packs for use as driving power sources in such electric tools are required to withstand exposure to high voltage and intense discharge. As a consequence of this trend, a single battery pack has come to have a larger number of cells, for example, as many as 30 pieces of cells. As a cell constituting a battery pack for use in electric tools, a nickel-cadmium rechargeable battery is commonly used, and besides a nickel metal hydride battery is recently coming into wider use in keeping with the demand for environmental conservation. These batteries are built as maintenance-free, cylindrical-shaped rechargeable batteries of enclosed-type.
In a conventional battery pack, a multiplicity of cylindrical rechargeable batteries needs to be housed in a limited space of a pack case. Thus, they are stacked in a staggered arrangement to make efficient use of available space. The electrodes of the batteries are electrically connected in series or parallel with one another by a nickel terminal plate or the like.
As described above, an enclosed-type battery is commonly used for a battery pack. In an enclosed-type battery, however, during charging and discharging operations, heat of reaction due to gas absorption reaction occurs along with generation of Joule heat. This leads to a temperature rise in the battery. Furthermore, a battery pack usually employs a resin-made pack case exhibiting poor thermal conductivity so as to be electrically insulated from a cell. Therefore, when a multiplicity of cells housed in the pack case generate heat, the pack case of poor thermal conductivity prevents heat from dissipating to the outside. Further, in a case where the battery pack is used as a driving power source of an electric tool or the like, during discharging for driving a motor or other, a large current is generated. This also leads to a temperature rise in the cell. For these reasons, the temperature of the cell might rise up to 80° C. or above. In this case, an alkaline rechargeable battery suffers from degradation of charging and discharging characteristics and shortening of charging and discharging cycle life, because it is prevented from being fully charged under high temperature conditions.
Moreover, of the cells housed in the pack case in a staggeredly-stacked arrangement, particularly the ones in the middle exhibit poor thermal dissipation characteristics due to their adjacent cells, and thus suffer from a more significant temperature rise as compared with the other cells. The difference in temperature among the cells leads not only to deterioration of the cells but also to variation in battery performance, as well as variation in the extent of deterioration, among the cells.
To suppress a temperature rise in staggeredly-stacked cells housed in a battery pack, various proposals have been made to date. For example, in the battery pack disclosed in Japanese Patent Publication No. 9-306447, a wave-like partition plate made of synthetic resin containing metallic oxide is interposed between tiers of cells. Heat generated by each cell is collected by the wave-like partition plate and is then dissipated, through a case cover making contact with the end portion of the wave-like partition plate, to the outside. Moreover, in the battery pack disclosed in Japanese Patent Publication No. 6-223804, of a plurality of staggeredly-stacked cells, the ones in the middle each have a heat dissipating plate disposed forward of its electrode. The heat dissipating plate has a terminal plate portion formed integrally therewith. The terminal plate portion is connected only to the electrode of the centrally-located cell. In this way, the difference in temperature among the cells is minimized. However, in either of these battery pack structures, the effect of collecting heat generated by the cells and the effect of dissipating the heat out of the case are insufficient. In addition, heat generated by the staggeredly-stacked cells is not evenly collected. This makes it impossible to narrow the difference in temperature among the cells.
The present invention has been made in view of the above-described problems with conventional art, and accordingly its object is to provide a battery pack in which a rise in temperature of a cell due to heat generation is suppressed and the difference in temperature among the cells is minimized.